1. Field of the Invention
This invention relates to a pulse-width modulation circuit suitable for amplification of audio signals. and more particularly to the improvement of such a pulse-width modulation circuit.
2. Description of the Prior Art
As one type of audio amplifiers, a pulse-width modulation amplifier is known in the art, wherein an audio signal to be amplified is temporarily converted into a pulse signal by a pulse-width modulation circuit, and thereafter it is amplified in the form of a pulse signal in order to effectively attain a desired amplification. FIG. 1 shows a block diagram illustrating one example of such conventional amplifiers. In the figure, reference number 1 designates an input terminal to which an audio signal e.sub.a to be amplified is applied, and reference number 2 generally designates a pulse-width modulation circuit. The pulse-width modulation circuit 2 comprises an oscillator 2a and a modulation section 2.sub.b. The oscillator 2a generates a carrier signal e.sub.c having a constant frequency or period T, such as a square wave signal. The modulation section 2.sub.b converts the audio signal e.sub.a into a pulse signal e.sub.p which has a duty factor corresponding to a signal level (amplitude) of the audio signal e.sub.a. Pulse signals e.sub.p thus obtained are illustratively shown in FIGS. 2 (a) and ( b). The pulse signal e.sub.p of FIG. 2 (a) shows a waveform which is obtained during the time when the audio signal level is zero, that is, at a 0% modulation factor, while the pulse signal e.sub.p of FIG. 2 (b) shows a waveform which is obtained during the time the audio signal level is high. Thus, the pulse signal e.sub.p changes its duty factor in accordance with the audio signal level. The pulse signal e.sub.p is then power-amplified at a pulse amplifier 3, and is thereafter applied to a low-pass filter 4 to remove therefrom signal components corresponding to the carrier signal e.sub.c and to obtain a demodulated signal having signal components corresponding only to audio signal e.sub.a. Then, the demodulated signal is applied to a loudspeaker 5.
In the conventional pulse-width modulation circuit 2 described above, modulation is performed by employing a carrier signal e.sub.c whose constant frequency such as 500 kHz (or constant period such as 2 .mu.sec) is determined depending upon the highest possible frequency of the audio signal e.sub.a such as 15 kHz. In order to amplify or transmit the waveform of the pulse signal e.sub.p with high fidelity, it is necessary for the associated circuitries to have a sufficient band width for performing such operation. For example, in the case of the pulse signal e.sub.p of FIG. 2 (a) at 0% modulation, since the pulse signal e.sub.p has low and high level durations both having the same time period T/2 and has a comparatively small amount of higher order frequency components, a moderate band width (such as several MHz for T=2 .mu.sec) may suffice for the circuit operation. On the other hand, in the case of the pulse signal e.sub.p of FIG. 2 (b) resulted from a higher modulation, since the pulse signal e.sub.p has a high level duration with a longer time period and a low level duration with a shorter time period (or vice versa) and has a relatively large amount of higher order frequency components, a band width at least about ten times broader as that at a 0% modulation is required. As a result, if the conventional pulse-width modulation circuit is used, the pulse-width modulation circuit itself as well as the circuitries for amplifying or transmitting the pulse signal must be implemented with expensive high speed switching elements and with circuit arrangements having a broader band width. Thus, a very expensive and complicated circuit must have been employed. Furthermore, it is practically very difficult to realize such a wide band width so that the distortion factor is inavoidably degraded.